A galvannealed steel sheet has recently been in widespread use in various sectors of industry such as automobiles, electric home appliances, construction materials, and the like because of their excellent weldability, paintability, corrosion resistance, economic merits, and the like. A high strength galvannealed steel sheet having good press formability is also demanded from the viewpoint of promoting safety and weight reduction of automobiles. Therefore, the galvannealed steel sheet is required which can meet all of the aforesaid requirement.
Normally, the galvannealed steel sheet is manufactured by heating up a hot--dip galvanized steel sheet to a temperature in the range of 500 to 600.degree. C. for a retention time of 3 to 60 seconds in a heating furnace for Fe--Zn alloying. By applying a Fe--Zn alloying treatment as above, a Zn layer composing an original metallic coating is turned into a Fe--Zn alloy layer containing normally 8 to 12 wt % of Fe. A coating weight of the metallic coating after the treatment, that is, a Fe--Zn alloy layer, is normally 20 to 70 g /m.sup.2 of the surface on one side of the steel sheet.
In application of the galvannealed steel sheet for manufacturing automobile body parts, such properties as powdering resistance and chipping resistance are important. Powdering is a phenomenon in which the metallic coating is broken into fine pieces and exfoliated at sites where the steel sheet is subjected to compressive deformation during press forming, and the like. Not only corrosion resistance is degraded at sites of the steel sheet where powdering occurs, but also fine pieces of the exfoliated coating, adhered to press dies, give rise to a cause for surface defects of a formed product. Various measures have been adopted for preventing powdering, including reduction in a Zn coating weight, restriction on Al concentration in a plating bath, restriction on Fe--Zn alloying conditions and Fe content of a galvannealed coating.
Chipping is a phenomenon in which the galvannealed coating exfoliates from the surface of a base metal, occurring, for example, when pebbles, and the like, collide with a running automobile, and the impact force of the pebbles is applied to the painted surface of the automobile body. Automobiles in service in cold environments are susceptible to the chipping phenomena.
Since both powdering and chipping are phenomena whereby the galvannealed coating exfoliates, it has been considered that enhancement in powdering resistance would be accompanied by improvement in chipping resistance. However, it has since been found that enhanced powdering resistance does not necessarily result in improved chipping resistance, and adhesion property at the interface between the base metal and the galvannealed coating needs to be enhanced in order to improve chipping resistance.
For example, a method of manufacturing a galvannealed steel sheet focusing on improvement in the adhesive property at the interface between the base metal and the galvannealed coating is disclosed in Japanese Patent Publication Laid-open (Kokai) No. Hei 2-97653. The steel sheet according to the aforesaid invention has a micro-structure formed by diffusion of Zn into the grain boundaries on the surface of a base metal. The steel sheet described above is manufactured by plating a base metal in a hot-dip galvanizing bath containing Al in a concentration set much higher than for normal cases, and by applying the Fe--Zn alloying treatment at higher temperature than for normal cases. However, use of a plating bath containing Al in higher concentration requires application of the Fe--Zn alloying treatment at higher temperature and for longer period of time than for normal cases. Powdering resistance tends to be impaired when the Fe--Zn alloying is processed at higher temperature and a longer processing time results in a poorer productivity.
P added steel is in widespread use for manufacturing a high strength steel sheet for use in automobiles, because the strength of a steel sheet can be increased at low cost by adding P. However, an improvement in chipping resistance of the galvannealed steel sheet with an increased P content has been difficult to achieve. This is due to the fact that with higher P content, reactivity of Zn in grain boundaries of the base metal is impaired. Consequently, the effect of improving coating adhesion resulting from diffusion of Zn into grain boundaries on the surface of the base metal can not be expected with respect to a steel with a high P content.
Japanese Patent Laid-open (Kokai) Publication No. Hei 6-81099 discloses a steel sheet having excellent coating adhesion by holding down P content detrimental to chipping resistance at 0.007 wt % or lower, and by roughing the surface of the base metal at its boundary with the galvannealed coating. However, with said steel, Si and Mn are used in place of P to increase the strength. It is not a desirable means to increase Si and Mn contents as a substitution for limiting P content lower from the viewpoint of increasing tensile strength of the base metal economically.
It is reported in GALVATEC '95 CONFERENCE Proceedings (September 1995), p. 343 to 353 and p. 753 to 759, that coating adhesion at an interface between a base metal and a metallic coating is enhanced when Si is added to an ultra-low carbon steel with Ti added thereto because diffusion of Zn into grain boundaries of the base metal is promoted. However, the technology disclosed therein is intended for application to a soft ultra-low carbon steel while no mention was made of a P added steel sheet of high tensile strength, which is in great demand as steel sheet for use in automobiles.